Field of the Disclosure
Embodiments of the present invention generally relate to a method and apparatus for capacitive touch sensing, and more specifically, to filtering a received signal.
Description of the Related Art
Input devices including capacitive touch sensing devices (also commonly called touchpads, touchscreens, or touch sensor devices) are widely used in a variety of electronic systems. Capacitive touch sensing devices may be used to provide a graphical user interface (GUI) for an electronic system. For example, capacitive touch sensing devices are often used as input devices for larger computing systems (such as opaque touchpads integrated in, or peripheral to, notebook or desktop computers). Capacitive touch sensing devices are also often used in smaller computing systems (such as touch screens integrated in mobile phones).
A capacitive touch sensing device typically includes a sensing region, often demarked by a surface, in which the capacitive touch sensing device determines the presence, location and/or motion of one or more input objects. Inherently, the sensing region must be exposed to touch, which exposes it to external interference, which is large compared to the sensitive measurements of capacitive touch. Therefore, signal conditioning such as filtering is very important to the quality of the measurements. External interference may extend to arbitrarily high frequencies, which may be above the Nyquist limit which is set by the sampling rate of discrete-time components such as an analog-to-digital (ADC) converter, resulting in aliasing. What is needed is at least some continuous-time analog anti-alias filtering applied ahead of any sampling, to remove interference frequencies that would alias, preferably without using large component values.